1. Field of the Invention
The invention relates to an apparatus for separating and carrying off the topmost sheet from a stack of sheets. A support is provided for supporting the stack of sheets. A sheet transport member is disposed above the stack and the bottom of the transport member is provided with air passages with different air flow resistances. A positioning member holds the sheet transport member at a short distance above the stack. An air displacement member generates an upwardly directed airstream through the air passages.
2. Description of Background Art
An apparatus of this kind is set forth in U.S. Pat. No. 5,150,892. The apparatus described therein has a sheet transport path with a suction box surrounded by four porous conveyor belts disposed near one another. The suction box is provided with air passage apertures through which an upwardly directed air stream can take place to suck the topmost sheet against the conveyor belts. In this known apparatus, the air passages in the case of a sheet which is not sucked into contact are closed by a valve at the outermost conveyor belts so that when the air flow is activated the topmost sheet is initially sucked against the middle conveyor belts, so that the negative pressure in the suction box increases and the valves at the outermost air passages open, with the result that parts of the sheet situated next to the part of the sheet which has already been sucked into contact are sucked against the suction box.
A disadvantage of this known apparatus is that a relatively considerable force is required in order to pull the topmost sheet loose first in the middle of the stack. This is produced by the fact that sucking the topmost sheet up results in a strong negative pressure in the middle beneath the topmost sheet, and this necessitates an even stronger upwardly suction force from the sheet transport means in order to pull the topmost sheet loose.
If the negative pressure between the sheets is greater between the topmost sheet and the sheet directly therebeneath than between the sheets under the topmost sheet, then there is a considerable risk that the two topmost sheets will be sucked together against the sheet transport means.
A disadvantage of first sucking a sheet in the middle and only then at its sides, is that the curvature of a sheet in the middle requires an extra high suction force if the sheet for sucking is stiff and/or can slide only with difficulty over the sheet therebeneath.